How to obtain the `ty::Const` of a promoted constant literal?

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Hi all,

Could someone explain to me how to evaluate a constant operand (mir::Operand::Constant), to the corresponding ty::Const?

In particular, I don't know how to handle the case of a promoted constant literal:

mir::Constant{ literal: mir::Literal::Promoted { index }, .. }

The tcx.const_eval(..) method seems to do what I want, but I don't know how to get the def_id of the constant that I'm considering. That is, to build the GlobalId argument it seems that I need a ty::Instance, and to build it I need the def_id of the constant literal.

Edit: to add a bit of context, the mir::Operand::Constant that contains the promoted constant literal comes from an assignment in the MIR of a procedure that I'm analyzing.